Currently, III-V compound based photovoltaic devices are epitaxially grown on substrates and remain affixed thereto throughout fabrication and deployment as a solar cell. In many cases, the substrates can be approximately 150 μm thick. Having substrates with such thickness may introduce a number of undesirable consequences for a solar cell.
One such undesirable consequence is weight. The thick substrate can make up a large percentage of the overall weight of the resulting solar cell. In certain applications, such as space applications, weight and size of a solar cell can be significant given the liftoff capability of the selected launch vehicle.
Another undesirable consequence is poor thermal conductivity. The substrate increases the thermal impedance between the solar cell and a heat sink on which the substrate and solar cell may be mounted. The increased thermal impedance results in higher junction temperatures in the solar cell, which, in turn, reduces the efficiency of the solar cell.
Another undesirable consequence is environmental impact. The substrate serves no purpose other than as a mechanical support for the solar cell. In addition, to achieve a substrate thickness of approximately 150 μm, it is typically necessary to remove, mechanically or chemically, part of the substrate, which amounts to further waste.
Another undesirable consequence is lack of flexibility. A 150 μm thick substrate is rigid, which means that the solar cell cannot be mounted on a curved surface and cannot be rolled up for easy storage.
Although, it is desirable that the substrate of a solar cell be as thin as possible to reduce the weight and to increase the thermal conductivity, thin substrates can also present undesirable difficulties. If the substrate is too thin, the III-V compound solar cell can become so fragile that it is very difficult to handle. For example, a layer of Gallium Arsenide (GaAs) that is 100 mm in diameter, but only 2-10 μm thick, tends to crack and break when subjected to even very gentle handling. This consideration applies particularly to whole wafers of III-V compound solar cells. That is, the thinner the substrate, the more difficult it becomes to fabricate whole wafers of III-V compound solar cells without a decrease in yield due to breakage and handling damage.
Accordingly, a thin film III-V compound solar cell and methodologies for fabrication of thin film III-V compound solar cells that utilizes a removable and reusable substrate in the fabrication process is desirable.